Method for combustion of hydrocarbons

ABSTRACT

In order to enrich natural gas or other hydrocarbon gases with hydrogen, thus reducing the carbon content and thereby achieving a reduction or elimination of the discharge of carbon dioxide during combustion of the gases, a pyrolytic process is conducted in the feed stream for the natural gases or hydrocarbon gases which are to be burned. The gas is passed through a reactor in which it is decomposed at least partially into a carbon constituent and a hydrogen constituent. The carbon constituent is removed to a desired level. Any remaining constituent together with the hydrogen constituent is conveyed to the combustion process while the removed carbon constituent is conveyed out of the process for separate application. The pyrolytic process can be carried out on the entire stream or only a partial stream.

This is a continuation-in-part of application Ser. No. 08/244,298, filedon Sep. 22, 1994, which is now abandoned.

FIELD OF THE INVENTION

The invention concerns a method for reducing the carbon content in afeed flow of natural gas or other hydrocarbon gases for a combustionprocess or a chemical process in order thereby to eliminate to thedesired degree or at least to reduce the discharge of carbon dioxides inthe combustion of the gas, e.g. in connection with a gas-fired powerstation.

BACKGROUND OF THE INVENTION

During the combustion of fossil fuels, coal, natural gas and otherhydrocarbons, both water in the form of vapour and carbon dioxide willbe found in the waste gases. The proportional incidence between thesetwo gases will be dependent on the quantitive ratio of carbon tohydrogen in the fuel concerned. Coal will produce almost only carbondioxide, methane will produce water and carbon dioxide in the ratio of2:1, while hydrogen will only produce water. In the efforts to reducethe greenhouse effect, the discharges of carbon dioxide are becomingever less acceptable. Thus it is vitally important to limit thesedischarges, the best best solution being to eliminate them entirely.

SUMMARY OF THE INVENTION

The object of the present invention is to improve this situation byproviding a method for reducing the carbon content of natural gas andhydrocarbon gas. This reduction should be capable of being implementedin such a manner that a desired degree of reduction of the CO₂ contentcan be achieved after a combustion or a chemical process. The reductioncan extend all the way through to the use of pure hydrogen as a fuel,thus completely avoiding the discharges of CO₂.

Thus a further object of the invention is also to make it possible fornatural gas to be used as a fuel with a greatly reduced production ofcarbon dioxide, e.g. in a gas turbine.

These objects are achieved by a method which according to the inventionis characterized by the features in the claims presented.

The invention is based on the fact that it is possible to decomposehydrocarbons pyrolitically into carbon and hydrogen. By using purehydrogen as a fuel the discharge of carbon dioxide will be eliminated.If carbon is removed from the natural gas or the hydrocarbon gas whichis used as a fuel, the discharge of carbon dioxide will be reducedbefore the combustion.

A reduction of this kind can be carried out by performing a complete orpartial conversion of natural gas or hydrocarbon gases in a feed streamto a combustion process or a chemical process. The feed stream isdecomposed to the desired degree into carbon and hydrogen, whereof thenatural gas with the reduced carbon content goes to combustion or achemical process and the carbon constituent is removed from the processfor separate application. Such a reduction can also be performed byconverting a partial stream of the natural gas or the hydrocarbon gas.

In both cases the decomposition into carbon and hydrogen will requireenergy. The energy content in the gases with reduced carbon content islower than in pure natural gas or hydrocarbon gas since the liberatedcarbon represents the lost energy. The result will be that more gas intotal has to be supplied to the process in order to achieve the same neteffect as from pure natural gas or hydrocarbon gas. The extent of thisadditional amount will be dependent on the degree to which the carboncontent has to be reduced, i.e. the extent to which a conversion has tobe performed, and also on the efficiency of the combustion processconcerned.

The object is to be able to use pure hydrogen as a combustion gas inorder thereby to be able to completely eliminate the CO₂ discharges.

At existing plants, however, there will be operating conditions whichprevent the use of pure hydrogen, thus making it necessary to add acertain amount of hydrocarbons.

The process is therefore designed so as to offer the possibility ofadding a certain amount of natural gas or hydrocarbon gas to thehydrogen stream and the amount can be adjusted according to theoperating conditions.

After a combustion process, the waste gases are purified by methodsknown in the art and the purification methods are often extremelyexpensive, as for example in the case of catalytic purification.

It has now been discovered that the expenses involved in the conversionof natural gas or hydrocarbon gas which are used as a fuel for acombustion plant, i.e. before combustion, can be compensated for, thusenabling an economically justifiable process to be achieved despite theenergy loss in the combustion stream. This is achieved by producing purecarbon in addition to hydrogen in the pyrolytic process in the methodaccording to the invention. The carbonaceous material will be present inthe form of carbon black and as such a material will have a very highvalue. Carbon black can, e.g., be used as a reduction agent in themetallurgical industry or on the conventional "carbon black market". Ifthe carbonaceous material is used for anodes in the aluminium industry,this will entail substantial improvements both from the environmentaland the economic point of view, since the new anodes will not polluteeither the electrolyte and thus the product or the environment by thedischarge of sulphur and tar.

Thus by means of the invention a highly favourable combination has beenobtained of financially profitable industrial production andconservation of the environment, i.e. a reduction in pollution.

DETAILED DESCRIPTION OF THE INVENTION

In the following section the invention will be described in more detailby means of an example of the use of the method.

As an example, the operation of a gas-fired power station based onnatural gas as its energy source has been chosen. The example isintended to illustrate the main principles of the invention. Otherapplications of the invention could be the production of pure hydrogenand the use of the method in all processes where natural gas or anotherhydrocarbon are used as the energy source. The invention is particularlywell suited in connection with power production with fuel cells based onhydrogen as the means of combustion.

In the supply line to a gas-fired power station, which, e.g., isoperated by the use of natural gas, e.g. methane, there is provided areactor which is operated according to the pyrolytic principle with aplasma torch and which causes a decomposition of the incoming naturalgas or hydrocarbon gas into a constituent of hydrogen and a constituentof carbon particles. The reactor may be located in a branch line or aline bypassing the reactor. The method of operation of such a reactorand the design of a plasma torch for this are described in theapplicant's related U.S. patent application, Ser. No. 08/244,295 filedMay 26, 1994 now U.S. Pat. No. 5,486,674 granted Jan. 23, 1996 thedisclosure of which is incorporated herein by reference. As described inthese documents, a quality control can be performed for the carbonconstituent. The hydrogen formed is also used via a return line as aplasma-forming gas for the torch, thus enabling this entire reactor tobe operated without causing any pollution.

The hydrogen gas from the reactor is then passed to the gas turbinewhere it is used either alone or together with hydrocarbons (methane) asfuel in the gas turbine. The gas turbine is operated in a conventionalmanner. However, there is one major difference, which is that thedischarge of carbon dioxide is reduced or completely eliminated. Thedegree to which the discharges of carbon dioxide are reduced will bedependent on how large a proportion of the supply to the gas turbine iscomposed of pure hydrogen. Throughout the process, the proportion can beregulated in the entire area from no admixture of hydrogen to purehydrogen.

What is claimed is:
 1. A method of reducing the discharge of carbondioxide from a process using the combustion of natural gas orhydrocarbon gas to generate energy, the steps comprising, prior to theprocess for combustion, feeding substantially all of the said gas to adecomposition reactor including a plasma torch to subject said gas to apyrolytic process of the type that yields hydrogen and undecomposed gasand then removing the carbon from the hydrogen and the undecomposed gasso that the gas stream exiting the decomposition reactor issubstantially hydrogen and undecomposed gas.
 2. The method as claimed inclaim 1 including the step of recycling hydrogen produced in thedecomposition reactor to the decomposition reactor plasma torch.
 3. Amethod of reducing the discharge of carbon dioxide from a process usingthe combustion of one of natural gas and hydrocarbon gas to generateenergy, the steps comprising, prior to the process for combustion,feeding the entire amount of the gas to the inlet of a decompositionreactor and through the said reactor to the outlet of the reactor, saiddecomposition reactor including a plasma torch and including the step ofsubjecting said gas to a pyrolytic process with said plasma torch toyield hydrogen, carbon and undecomposed gas and then removing saidcarbon from the hydrogen and undecomposed gas so that the gas streamexiting the outlet of aid decomposition reactor is substantiallyhydrogen fuel and feeding the hydrogen fuel and undecomposed gas to acombustion process.